1. Field of the Invention
This invention relates generally to a tone arm assembly, and is directed more particularly to a novel tone arm assembly which is controlled with an electro-magnetic force.
2. Description of the Prior Art
In the recent development of phonograph record players, there is an effort to reproduce a signal from a record disc with high fidelity. In general a cartridge, which will trace the sound grooves of the record disc, is attached to a tone arm which is pivoted at its middle point. This tone arm is formed of a rigid pipe made of, for example, aluminium, so that when the stylus of the cartridge traces the grooves of the record disc, the tone arm is vibrated at a low frequency in the parallel (horizontal) and perpendicular (vertical) directions with respect to the plane of the record disc. This vibration is known as a low frequency resonance in the horizontal direction and a low frequency in the vertical direction, and the frequency of the resonance vibrations is on the order of 7 to 10 H.sub.z. Such low frequency resonances of the tone arm act to add a modulation component to the signal reproduced by the cartridge, so that the prior art tone arm can not reproduce a sound signal from the record disc with high fidelity.
A tone arm assembly recently proposed is of the balanced type. That is, the tone arm is provided at its one end with a cartridge and at its other end with a counter weight, and in this case the counter weight is manually adjusted in accordance with the cartridge so that the stylus of the cartridge contacts the groove of a record disc at an optimum pressure. This adjustment is divided into adjustments of horizontal balance, zero balance and stylus pressure of the tone arm. In general, many of these adjustments are carried out by manually varying the position of the counter weight as described above. Since the above manual adjustment is rather troublesome, an adjustment by using electro-magnetic force is proposed in, for example, the U.S. Pat. No. 3,830,505. According to this U.S. patent, an adjustment of a stylus pressure is possible but the concept of avoiding a low frequency resonance of a tone arm is not disclosed therein.
Further, since the groove of a record disc is spiral, in the case that the record disc is reproduced by a tone arm with an offset angle, an inside force is generated in the tone arm. Thus, an inside force canceller is provided in connection with the tone arm so as to cancel the inside force. In the art, many of the inside force canceller mechanisms are a coil spring, a lever and so on all of which utilize mechanical force, so that their accuracy is deteriorated over a long period of use of the tone arm.